Pneumatically operated self-propelled soil penetrating machines are widely used for trenchless installation of cables and pipes, and for similar purposes. Typically these machines comprise as major assemblies a tubular housing, an air distributing mechanism, a striker, and a chisel. The striker cyclically reciprocates Inside of the tubular housing. The motion of the striker is caused by the compressed air. A working cycle of the machine consists of a forward and backward strokes of the striker. The majority of these machines can work in forward and also in reverse modes of operation. The penetration of the machine into the soil occurs during the forward mode of operation. In this mode of operation the striker at the end of its forward stroke imparts a blow to the chisel. This results in an incremental penetrating of the machine into the soil. In the reverse mode of operation the striker at the end of its backward stroke imparts a blow to an appropriate component that is rigidly connected to the tubular housing. As a result of this the machine incrementally moves backward in order to get out from the hole.
Numerous US patents describe the designs of these machines. The following US patents represent some examples of certain design features of these machines: U.S. Pat. No. 3,651,874 (March 1972); U.S. Pat. No. 3,708,023 (January 1973); U.S. Pat. No. 3,737,701 (April 1973); U.S. Pat. No. 3,744,576 (July 1973); U.S. Pat. No. 3,756,328 (September 1973); U.S. Pat. No. 3,865,200 (February 1975); U.S. Pat. No. 4,078,619 (March 1978); U.S. Pat. No. 4,214,638 (July 1980). An analysis of some patents can be found in U.S. Pat. No. 5,031,706 (July 1991) and U.S. Pat. No. 5,336,487 (July, 1993) issued to Spektor (the author of the present invention). It is important to emphasize that all currently existing on the market machines incorporate the same design concept of the machine that is described in U.S. Pat. No. 3,651,874 that is issued in March 1972 to Sudnishnikov et all. All related patents issued later offer certain improvements without any change in the concept of the structural design that is presented in the above mentioned U.S. Pat. No. 3,651,874. One of the disadvantages of the machine according to the U.S. Pat. No. 3,651,874 is associated with its air distributing mechanism that severely limits the possibility to increase the length of the striker's stroke. Actually, the air distributing mechanism according to the U.S. Pat. No. 3,651,874 supports relatively short strokes with no possibility of any essential increase of the stroke. The reason for this is associated with the design specifics that control the delivery of compressed air for the backward stroke. Without considering the details, it should be emphasized that in the existing machines, the backward stroke of the striker is caused by an injection of compressed air, not by a continuous flow of compressed air. In general, an injected portion of compressed air can support just a limited backward stroke. The air distributing mechanism, according to the above mentioned patent, is designed in such a way that a significant part of the cross-sectional area of the striker is constantly subjected to the action of the nominal air pressure to enable the forward stroke and in the same time to resist to the backward stroke. This results in a relatively short backward stroke. The importance of the length of the stroke for the improvement of the efficiency of the machine is presented below.
The length of the stroke determines the striker's kinetic energy which is equal to the product of multiplying the compressed air force by the length of the stroke. The nominal pressure of the air is predetermined by the norms of the industrial compressors. Thus, for a certain machine the only parameter that could change the kinetic energy of the striker is the length of its stroke. The sum of the lengths of the striker and its forward stroke (for the forward mode of operation) can be considered as the effective length of the tubular housing.
The efficiency of the machine in the forward mode of operation is proportional to the kinetic energy that the tubular housing (including all associated parts) obtains as a result of an impact of the striker. The most important characteristic of the machine is its efficiency during the forward mode of operation. In the proposed invention the concept of optimization of the parameters of the machine is developed for the forward mode of operation. However the optimization of the parameters results in improvement of the reverse mode of operation as well.
Thus, the shorter the length of the striker the longer is its forward stroke, and as a result of this, the higher impact energy the striker possesses before the impact (and vice versa). The kinetic energy of the tubular housing depends on the amount of impact energy of the striker and of the level of energy transfer from the striker to the tubular housing. The level of energy transfer depends on the mass ratio between the striker and the tubular housing (while other factors like hardness, shape, etc being equal). The smaller the mass of the striker the lower is the level of energy transfer, and vice versa. So, a short striker will possess a high impact energy, but the energy transfer will be low, and vice versa. It becomes to be a problem of optimization that may reveal the existence of an optimal value of the length of the striker (or of the lengths of the forward stroke) with respect to maximum kinetic energy that the tubular housing (with the related parts) possesses after the striker imparts a blow to the chisel.
The author of this invention carried out an appropriate analytical investigation of the dynamics of the forward stroke of the striker and the process of energy transfer to the tubular housing. This investigation has revealed the existence of optimal values of the lengths of the striker and its forward stroke with respect to the maximum value of kinetic energy of the tubular housing (with associated parts) obtained as a result of the striker's blow to the chisel. This analytical investigation and its results are not published, however they can be obtained from the author by demand. The existence of optimal values of the lengths of the striker and its stroke with respect to the maximum value of the kinetic energy of the tubular housing (with the associated parts) was not known before. The formulas for calculating the optimal values of the lengths of the striker and its forward stroke are presented in the specification.
The calculations based on this investigation show that the optimal value of the length of the striker is always shorter than the optimal length of the forward stroke, and, therefore, the optimal length of the striker is always less than 50% of the effective length of the tubular housing. In all existing machines the length of the striker is longer than the length of its forward stroke. These calculations also show that a hypothetical optimized machine having the same effective length of the housing as an existing machine would have approximately 2.5 times more kinetic energy per cycle. Actually, the length of the striker's forward stroke of the existing machines does not exceed 25% of the effective length of the tubular housing. Thus, all existing pneumatically operated self-propelled soil penetrating machines are characterized by extremely low efficiency in comparison with the hypothetical optimized machines.
As it was mentioned above, the air distributing mechanisms of the existing machines impose very strict limitations on the increase of the stroke. It is problematic for these machines to increase the stroke even by a few percents.
U.S. Pat. No. 7,273,113 B2 (September 2007), issued to the author of the current invention, describes a soil penetrating machine which is characterized by a long stroke air distributing mechanism. Actually, this machine does not impose limits on the length of the striker's stroke. However, this machine cannot function if the stroke's length considerably exceeds 50% of the effective length of the tubular housing. This can be explained considering a hypothetical machine having the striker shorter than the stroke. During the functioning of the machine the compressed air is cyclically exhausting to the atmosphere through a radial exhaust passage (hole) in the wall of the tubular housing. The distance between the internal forehead surface of the chisel and the exhaust hole is a little longer than the length of the striker. This allows to the striker to overlap the exhaust passage during its forward, while just at the very end of the forward stroke (before the blow) the exhaust passage becomes not overlapped allowing the compressed air behind the striker to escape to the atmosphere. As a result of this, the air distributing mechanism redirects the flow of the compressed air into the space in front of the striker, forcing it to begin its backward stroke. The striker instantly overlaps the exhaust passage, which remains overlapped during the entire backward stroke for the machine according to the U.S. Pat. No. 7,273,113 B2. This can happen if the length of the striker exceeds 50% of the effective length of the tubular housing (and, obviously, longer than its stroke). In a hypothetical case, if the length of the striker is essentially less than 50% of the mentioned effective length, the striker will be not able to overlap the exhaust hole all the time during its backward stroke (because the striker is too short), and the exhaust hole will become open to the atmosphere before the striker will complete its backward stroke. Hence, the compressed air will escape, and as a result of this the functioning of the machine will be terminated.
Thus, the existing air distributing systems of the pneumatically operated soil penetrating machines do not allow for the optimization of their parameters in order to achieve the maximum efficiency of their performance. The main reason for this is, first of all, that the structure of the existing machines does not allow to increase the striker's stroke in a considerable way. However the machine according to the U.S. Pat. No. 7,273,113 B2 (September 2007), issued to the author of the current invention, allows long strokes. But this machine also cannot be optimized due to exhaust issues explained above. The proposed invention represents an optimized pneumatically operated self-propelled reversible soil penetrating impacting machine that is characterized by maximum efficiency of performance in the forward mode of operation.